Since PGs exhibit various important physiological actions in minor amounts, active research works have been made thereon for the intended application to drugs. With respect to the extension of PG analogues, it has been attempted in various ways to extend .alpha. and .omega.-chains attached to the five-membered ring moiety. One method capable of free choice of .alpha. and .omega.-chains and efficient introduction is a two-component connecting method. More particularly, direct synthesis is possible by 1,4-addition of a zinc reagent of .alpha.-chain to an enone as shown by Scheme 1 (see H. Tsujiyama, N. Ono, S. Okamoto, F. Sato, Tetrahedron Lett., 31, 4481 (1990)). ##STR1##
In the synthesis of an intermediate having a double bond in its .alpha.-chain, however, it is synthesized by introducing an .alpha.-chain into an enone using a vinyl lithium reagent and then effecting functional group conversion as shown by Scheme 2. ##STR2##
The reason is as follows. It is impossible to form an organic zinc reagent of an .alpha.-chain because reaction conditions are severe and the stereo-chemistry of olefin is not maintained. An .alpha.-chain is synthesized as an alcohol-protected one because a lithium reagent having an ester group cannot be synthesized. Many stages are thus necessary (see S. Okamoto, Y. Kobayashi, H. Kato, K. Hori, T. Takahashi, J. Tsuji, F. Sato, J. Org. Chem., 53, 5590 (1988)).
Also the route developed by E. J. Corey et al. (Scheme 3) entails much decomposition and hence, low yields since Jones oxidation under severe conditions is utilized. It is industrially inadequate since a heavy metal (Cr) is used (see YAMAMOTO Shozo et al., "Prostaglandins and Related Physiologically Active Compounds", Kodansha, 1981). ##STR3##
Further, a silyl substituted derivative of formula (VIII) which can be readily synthesized according to the present invention as will be described later is useful in the synthesis of a 6-keto-prostaglandin which now draws attention as a ulcer drug. In the past, an intermediate of this 6-keto-PG has been synthesized through the following route. This technique, however, is not satisfactory as a practical production process since only a racemic modification is obtained and further optical resolution is needed to obtain an optically active substance (see M.
Brawner Floyd, Synthetic Communications, 4 (6), 317-323 (1974)). ##STR4##